Heretofore, an NDIR gas concentration meter has been used as an infrared gas concentration meter for measuring the gas concentration in the atmosphere. Taking advantage of the fact that the absorbable wavelength of infrared ray (IR) differs depending on a gas type, the NDIR gas concentration meter measures the gas concentration through detection of the absorption amount. The NDIR gas concentration meter uses a filter, which transmits only an infrared ray having a wavelength corresponding to a detection target gas, and an infrared sensor in combination, and measures the gas concentration by measuring the absorption amount.
Here, an NDIR gas concentration meter which is reduced in size, highly accurate and capable of performing stable measurement in various environments is in demand. As an NDIR gas concentration meter of this kind, an infrared gas analyzer that measures the gas concentration in the atmosphere or the like by use of a wavelength selective infrared detection element has been proposed (refer to Patent Literature 1, for example).
Patent Literature 1 discloses an infrared gas sensor configured by integrally forming a wavelength selective filter selectively transmitting an infrared ray with a particular wavelength from a light source, and an infrared detector detecting the infrared rays that have passed through this wavelength selective filter. Specifically, Patent Literature 1 discloses an NDIR gas analyzer using a bolometer as an infrared sensor. The disclosed NDIR gas analyzer, however, employs a structure in which the infrared sensor is provided so as to float in the air inside a sealed chamber, and even requires vacuum sealing or inert gas sealing. Although Patent Literature 1 mentions that a quantum infrared detector is optionally usable, the literature neither discloses nor suggests a specific structure or embodiment for the quantum infrared detector.
In general, an infrared sensor is categorized into a thermal infrared sensor or a quantum infrared sensor. The thermal infrared sensor is a sensor that uses an infrared energy as heat and is an element that converts, into electric signals, effects (resistance change, capacitance change, electromotive force, spontaneous polarization) brought about by an increase in temperature when the temperature of the sensor itself rises due to the heat energy of the infrared rays. Such a thermal infrared sensor is categorized into a pyroelectric type (PZT, LiTaO3), a thermoelectromotive force type (thermopile, thermocouple) or a conductive type (bolometer, thermistor). The thermal infrared sensors of these types have no wavelength dependency in the sensitivity thereof and require no cooling. However, the response rate is low and the detection capability is not very high. Meanwhile, the quantum infrared sensor is a sensor using electrons and holes generated by light photons when a semiconductor is irradiated with infrared rays. Such a quantum infrared sensor is categorized into a photoconductive type (such as HgCdTe), a photovoltaic type (such as InAs) or the like. Such quantum infrared sensors have a wavelength dependency in the sensitivity thereof and have advantages that the sensitivity is high and the response rate is high. However, the quantum infrared sensors need to be cooled. Thus, the quantum infrared sensors are generally used with a cooling mechanism such as a Peltier device or a stirling cooler. Accordingly, it has been difficult to apply the quantum infrared sensors to the aforementioned NDIR gas sensors.
In addition, in a case of using a thermal infrared sensor, the following structure is employed for the purpose of blocking heat. Specifically, an optical filter transmitting infrared rays is bonded to an opening portion of a can package. An infrared detection element detecting the infrared rays having passed through this optical filter is housed inside the can package.
Meanwhile, as a thermopile sensor, an infrared sensor in which an infrared detection element is placed in a molding resin instead of using a can package for simplification and an improvement in durability has been proposed (refer to Patent Literature 2, for example). The infrared sensor described in Patent Literature 2 includes: a flat plate-shaped optical filter selectively transmitting infrared light in a specific wavelength range; an infrared detection element including, on one of the surfaces thereof, a detection element portion for detecting infrared light having passed through the optical filter; and a support body which is provided between the optical filter and the detection-element-formed surface of the infrared detection element, and which also bonds together the optical filter and the infrared detection element while securing a predetermined gap between the optical filter and the detection-element-formed surface.
Specifically, Patent Literature 2 discloses a structure to achieve a reduction in size and weight of an infrared sensor by employing a simplified configuration without using a can package, and by providing an optical filter while a predetermined gap is secured on the detection-element-formed surface of the infrared sensor. In addition, Patent Literature 2 discloses that the embodiment uses a thermopile as the infrared sensor and employs a hollow structure. Moreover, it is described that the gap is secured so as to avoid damage on the infrared detection element of the infrared element or a scratch on the contact surface thereof.
Moreover, the support body described in Patent Literature 2 is provided only for the purpose of securing the gap, and has a function to prevent unnecessary light that has not passed through the optical filter from entering the infrared detection element through the gap, and also a function to prevent a scratch on the contact surface of the optical filter or the infrared detection element or damage on the infrared detection element. Accordingly, the support body has no function to hold the optical filter or no function for packaging.
In contrast, as described later in FIG. 7, a quantum infrared sensor according to the present invention has a structure in which a detection element surface is provided inside a molding resin and a surface to be in contact with an optical filter is the rear surface of the substrate of the detection element. Thus, the structure in which the optical filter and the infrared sensor are in contact with each other with no gap formed therebetween as shown in FIG. 6 is preferably used. Accordingly, a further reduction in size and thickness can be achieved.
Moreover, as described above, a quantum infrared sensor is an element converting infrared rays into electric signals by use of a photoconductive effect, photovoltaic effect or the like, and is generally used while being cooled. A quantum infrared sensor operable at room temperature has also been proposed (refer to Patent Literature 3, for example). The quantum infrared sensor described in Patent Literature 3 includes: a compound semiconductor sensor portion that detects infrared rays by use of a compound semiconductor layer provided on a substrate and then outputs electric signals; and an integrated circuit portion that performs an arithmetic operation on the electric signals outputted from the compound semiconductor sensor portion. In addition, the compound semiconductor sensor portion and the integrated circuit portion are housed in the same package. With this configuration, the quantum infrared sensor is less affected by electromagnetic noise or heat fluctuation, is allowed to perform detection at room temperature, and is also made reducible in size of the module.
Further, a quantum infrared sensor including, on a substrate, a quantum photoelectric conversion portion operable at room temperature has been proposed (refer to Patent Literature 4, for example). In this quantum infrared sensor, the quantum photoelectric conversion portion is packaged with a filter by a sealing resin.
However, Patent Literatures 3 and 4 described above disclose quantum infrared sensors, but do not disclose anything about application of the quantum infrared sensors to a gas sensor.
In other words, each of Patent Literatures 3 and 4 described above disclose a quantum infrared sensor operable at room temperature and packaged by resin, but does not state or suggest that the infrared sensor can be used in an NDIR gas concentration meter in combination with an optical filter and a holding frame.
In this respect, Patent Literature 5, for example, discloses a gas sensor using a quantum infrared sensor. The gas sensor described in Patent Literature 5 is a gas sensor in which a measurement cell and a reference cell are arranged in parallel. In addition, an optical filter corresponding to a measurement target component gas and a filter rotational chopper are provided between the cells and the quantum infrared sensor to detect a component concentration of a sample gas on the basis of a comparison between transmission amounts of infrared rays directed to the cells.
Here, Patent Literature 5 discloses that the quantum infrared sensor is applied to a gas sensor. However, a reduction in size is difficult in this case because the filter rotational chopper is used. In addition, Patent Literature 5 does not disclose anything about a specific configuration achieving a reduction in size by forming the infrared sensor element and the optical filter into a module and enabling a stable measurement against disturbance changes such as changes in the flow amount and temperature of the gas to be measured.
Specifically, Patent Literature 5 discloses an NDIR gas analyzer using a photoconductive infrared detection sensor. This infrared gas analyzer can detect the concentrations of multiple component gases by use of a single infrared sensor and a rotational chopper. However, Patent Literature 5 does not describe anything about a quantum infrared sensor configured by using a holding frame provided with multiple quantum infrared sensors, multiple optical filters and through holes as in the case of the quantum infrared sensor according to the present invention.
The NDIR gas concentration meter using a thermopile element has a problem that, when the temperature or flow amount of a gas to be measured changes to a large extent, the temperature of the sensor changes to a large extent, thus causing a large fluctuation in the output of the sensor. This leads to a problem that the gas concentration meter cannot perform practical measurement when used under the situation described above.
Meanwhile, in order to deal with the aforementioned significant change in the temperature of the sensor, a conventional infrared sensor element employs a method for easing the phenomenon by using a can package to thermally block and stabilizes a detection portion. Specifically, an air gap is provided around the sensor element and is also turned into a vacuum state or filled with a gas having a small thermal conductivity, or a heat sink portion having a large heat capacity is attached. However, such a configuration makes the element more complicated in shape and also larger in size and weight while requiring a high working accuracy for packaging. Thus, use of the aforementioned method causes an increase in cost.
Moreover, instead of using a can package, one using a package formed of a molding resin or the like, and one including a filter directly attached onto a surface of an infrared element have been proposed as well. However, a problem with these is that, when a thermal infrared sensor element is used, insufficient thermal isolation inhibits a stable measurement when the temperature of or the flow amount of a gas to be measured changes to a large extent.
Meanwhile, in a case where a conventional quantum infrared sensor is used, a method to thermally stabilize the element by use of a large heat sink or to cool the element by use of a Peltier device or liquid nitrogen is used because a stable, high sensitivity cannot be obtained at normal temperature. Here, a can package is used as in the case of a thermal infrared sensor for the purpose of preventing condensation due to the cooling of the element, for the purpose of enclosing the element with a gas having a low heat conductivity such as Xe or Ne for suppressing heat conduction to outside, or for other purposes. As a result, such a configuration involves an increase in the size of the element or complication of the shape of the element while requiring a high working accuracy for packaging. Thus, there arises a problem causing an increase in cost.
The present invention has been made in view of the aforementioned problems. Thus, an object of the present invention is to provide a quantum infrared sensor for an NDIR gas concentration meter and a quantum infrared gas concentration meter using the same, the quantum infrared sensor having a small and simple device shape and also being capable of performing stable measurement against disturbance changes such as changes in the flow amount and temperature of gas to be measured.